U.S. patent number 10,841,018 [Application Number 16/594,582] was granted by the patent office on 2020-11-17 for communication system.
This patent grant is currently assigned to NOKIA TECHNOLOGIES OY. The grantee listed for this patent is NOKIA TECHNOLOGIES OY. Invention is credited to Marko Leinonen, Aarno Tapio Parssinen, Nuutti Tervo.
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United States Patent |
10,841,018 |
Tervo , et al. |
November 17, 2020 |
Communication system
Abstract
An apparatus comprising means for: applying beam shape
coefficients for obtaining a beam shape output for a linear part of
the beam formed by an array of transmitters, each transmitter being
associated with a respective transmit path comprising a respective
power amplifier; varying the output characteristics of at least one
transmit path to increase nonlinear characteristics between the
transmit paths to provide a beam shape for a non-linear part of the
beam; linearizing the output of the array by: determining
linearization coefficients for the array for digitally
predistorting the input signal to the array; and applying the
linearization coefficients to the signal input to the array; and
calibrating the transmitter array by: measuring error distributions
over at least two transmit paths; modifying the beam shape
coefficients to better obtain the beam shapes subsequent to the
linearization; and applying the modified beam shape coefficients to
the array of transmitters.
Inventors: |
Tervo; Nuutti (Oulu,
FI), Leinonen; Marko (Oulu, FI), Parssinen;
Aarno Tapio (Espoo, FI) |
Applicant: |
Name |
City |
State |
Country |
Type |
NOKIA TECHNOLOGIES OY |
Espoo |
N/A |
FI |
|
|
Assignee: |
NOKIA TECHNOLOGIES OY (Espoo,
FI)
|
Family
ID: |
1000005188316 |
Appl.
No.: |
16/594,582 |
Filed: |
October 7, 2019 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20200169332 A1 |
May 28, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
Nov 26, 2018 [GB] |
|
|
1819202.1 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B
17/104 (20150115); H04B 7/0617 (20130101); H04B
1/0475 (20130101); H04B 17/12 (20150115); H04B
2001/0425 (20130101); H03F 1/3247 (20130101) |
Current International
Class: |
H03F
1/32 (20060101); H04B 17/12 (20150101); H04B
17/10 (20150101); H04B 7/06 (20060101); H04B
1/04 (20060101) |
Field of
Search: |
;375/297 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102035076 |
|
Apr 2011 |
|
CN |
|
2 415 167 |
|
Oct 2017 |
|
EP |
|
3267579 |
|
Jan 2018 |
|
EP |
|
3 334 052 |
|
Jun 2018 |
|
EP |
|
WO 2010/008968 |
|
Jan 2010 |
|
WO |
|
WO 2010/038227 |
|
Apr 2010 |
|
WO |
|
Other References
Combined Search and Examination Report under Sections 17 and 18(3)
dated May 8, 2019 corresponding to GB1819202.1. cited by applicant
.
Tervo Nuutti et al.: "Digital predistortion of amplitude varying
phased array utilising over-the-air combining", 2017 IEEE MTT-S
International Microwave Symposium (IMS), IEEE, Jun. 4, 2017, pp.
1165-1168. cited by applicant .
Mar. 24, 2020 Extended Search Report issued in European Patent
Application No. 19211639.0. cited by applicant.
|
Primary Examiner: Puente; Eva Y
Attorney, Agent or Firm: Squire Patton Boggs (US) LLP
Claims
The invention claimed is:
1. An apparatus, comprising: at least one processor; and at least
one memory including computer program code; the at least one memory
and the computer program code configured to, with the at least one
processor, cause the apparatus at least to perform: applying beam
shape coefficients for obtaining a beam shape output for a linear
part of a beam formed by an array of transmitters, each transmitter
being associated with a respective transmit path comprising a
respective power amplifier; varying an output characteristics of at
least one transmit path to increase nonlinear characteristics
between the transmit paths to provide a beam shape for a non-linear
part of the beam; linearizing an output of the array of
transmitters by determining linearization coefficients for the
array of transmitters for digitally predistorting an input signal
to the array of transmitters, and applying the linearization
coefficients to the signal input to the array of transmitters; and
calibrating the transmitter array by measuring error distributions
over at least two transmit paths, modifying the beam shape
coefficients to better obtain the beam shapes subsequent to the
linearization, and applying the modified beam shape coefficients to
the array of transmitters.
2. The apparatus as claimed in claim 1, wherein the at least one
memory and the computer program code are further configured to
cause the apparatus to alternate between the linearizing and the
calibrating until the beam shape, having a mixture of linear and
non-linear components, is obtained.
3. The apparatus as claimed in claim 1, wherein the measuring the
error distribution over at least two transmit paths comprises
measuring any of a power distribution, an amplitude distribution,
or an envelope distribution over the at least two transmit
paths.
4. The apparatus as claimed in claim 1, wherein determining the
linearization coefficients comprises obtaining time domain samples
of the signals output by the array of transmitters.
5. The apparatus as claimed in claim 1, wherein the at least one
memory and the computer program code are further configured to
cause the apparatus to perform: comparing the measured error
distributions to a threshold value; and alternating between the
linearizing and the calibrating while the measured error
distributions are above the threshold value.
6. The apparatus as claimed in claim 1, wherein the at least one
memory and the computer program code are further configured to
cause the apparatus to perform: comparing the measured error
distributions to a threshold value; and performing the calibrating
when the measured error distributions are above the threshold
value; and performing the linearizing when the measured error
distributions are below the threshold value.
7. The apparatus as claimed in claim 1, wherein the at least one
memory and the computer program code are further configured to
cause the apparatus to perform: providing the linearization
coefficients to the calibrating; and using the linearization
coefficients for modifying the beam shape coefficients.
8. The apparatus as claimed in claim 1, wherein the at least one
memory and the computer program code are further configured to
cause the apparatus to perform altering the beam shape during the
operation of the apparatus based on at least one of differences in
nonlinearities between nonlinear transmit paths, interference level
surrounding the apparatus, received signal power levels, movement
of the apparatus relative to other apparatuses with which the
apparatus is communicating, a determined quality level of a
received signal, a direction of interference in a communicated
signal, a direction of transmission or convergence speed associated
with linearizing the output of the array of transmitters, and a
direction of transmission or convergence speed associated with
calibrating the transmitter array.
9. The apparatus as claimed in claim 1, wherein the at least one
memory and the computer program code are further configured to
cause the apparatus to perform: determining the non-linear part of
the beam by calculating any of an adjacent channel power, a total
radiated adjacent channel power, a signal to noise ratio, and a
signal to interference ratio.
10. The apparatus as claimed in claim 1, wherein the varying the
nonlinear characteristics between the transmit paths is configured
to modify the nonlinear characteristics by least one of altering
the phase of a phase shifter in at least one transmit path,
altering a gain of a variable gain amplifier in at least one
transmit path, and biasing of a power amplifier in at least one
transmit path.
11. A method, comprising: applying beam shape coefficients for
obtaining a beam shape output for a linear part of a beam formed by
an array of transmitters, each transmitter being associated with a
respective transmit path comprising a respective power amplifier;
varying an output characteristics of at least one transmit path to
increase nonlinear characteristics between the transmit paths to
provide a beam shape for a non-linear part of the beam; linearizing
an output of the array of transmitters by determining linearization
coefficients for the array of transmitters for digitally
predistorting an input signal to the array of transmitters, and
applying the linearization coefficients to the signal input to the
array of transmitters; and calibrating the transmitter array by
measuring error distributions over at least two transmit paths,
modifying the beam shape coefficients to better obtain the beam
shapes subsequent to the linearization, and applying the modified
beam shape coefficients to the array of transmitters.
12. The method as claimed in claim 11, further comprising
alternating between the linearizing and the calibrating until the
beam shape, having a mixture of linear and non-linear components,
is obtained.
13. The method as claimed in claim 11, further comprising:
comparing the measured error distributions to a threshold value;
and alternating between said linearizing and said calibrating while
the measured error distributions are above the threshold value.
14. The method as claimed in claim 11, further comprising:
comparing the measured error distributions to a threshold value;
operating the calibrating when the measured error distributions are
above the threshold value; and operating the linearizing when the
measured error distributions are below the threshold value.
15. A computer program embodied on a non-transitory
computer-readable medium, said computer program comprising
instructions for causing an apparatus to perform at least: applying
beam shape coefficients for obtaining a beam shape output for a
linear part of a beam formed by an array of transmitters, each
transmitter being associated with a respective transmit path
comprising a respective power amplifier; varying an output
characteristics of at least one transmit path to increase nonlinear
characteristics between the transmit paths to provide a beam shape
for a non-linear part of the beam; linearizing an output of the
array of transmitters by determining linearization coefficients for
the array of transmitters for digitally predistorting an input
signal to the array of transmitters, and applying the linearization
coefficients to the signal input to the array of transmitters, and
calibrating the transmitter array by measuring error distributions
over at least two transmit paths, modifying the beam shape
coefficients to better obtain the beam shapes subsequent to the
linearization, and applying the modified beam shape coefficients to
the array of transmitters.
Description
FIELD
The present application relates to a method, apparatus, and
computer program.
BACKGROUND
A communication system can be seen as a facility that enables
communication sessions between two or more entities such as user
terminals, base stations/access points and/or other nodes by
providing carriers between the various entities involved in the
communications path. A communication system can be provided, for
example, by means of a communication network and one or more
compatible communication devices. The communication sessions may
comprise, for example, communication of data for carrying
communications such as voice, electronic mail (email), text
message, multimedia and/or content data and so on. Non-limiting
examples of services provided comprise two-way or multi-way calls,
data communication or multimedia services and access to a data
network system, such as the Internet.
SUMMARY
According to a first aspect, there is provided an apparatus
comprising means for: applying beam shape coefficients for
obtaining a beam shape output for a linear part of the beam formed
by an array of transmitters, each transmitter being associated with
a respective transmit path comprising a respective power amplifier;
varying the output characteristics of at least one transmit path to
increase nonlinear characteristics between the transmit paths to
provide a beam shape for a non-linear part of the beam; linearizing
the output of the array by: determining linearization coefficients
for the array for digitally predistorting the input signal to the
array; and applying the linearization coefficients to the signal
input to the array; and calibrating the transmitter array by:
measuring error distributions over at least two transmit paths;
modifying the beam shape coefficients to better obtain the beam
shapes subsequent to the linearization; and applying the modified
beam shape coefficients to the array of transmitters.
The apparatus may further comprise means for alternating between
the means for linearizing and the means for calibrating until the
beam shape, having a mixture of linear and non-linear components,
is obtained.
Measuring the error distribution over at least two transmit paths
may comprise measuring any of: a power distribution, an amplitude
distribution, and/or an envelope distribution over the at least two
transmit paths.
Determining the linearization coefficients may comprise obtaining
time domain samples of the signals output by the array.
The apparatus may further comprise means for: comparing the
measured error distributions to a threshold value; and alternating
between said means for linearizing and said means for calibrating
while the measured error distributions are above the threshold
value.
The apparatus may further comprise means for: comparing the
measured error distributions to a threshold value; and operating
the means for calibrating when the measured error distributions are
above the threshold value; and operating the means for linearizing
when the measured error distributions are below the threshold
value.
The apparatus may further comprise means for: providing the
linearization coefficients to the means for calibrating; and using
the linearization coefficients for modifying the beam shape
coefficients.
The apparatus may further comprise means for altering the beam
shape during the operation of the apparatus based on at least one
of: differences in nonlinearities between nonlinear transmit paths;
interference level surrounding the apparatus; received signal power
levels; movement of the apparatus relative to other apparatuses
with which the apparatus is communicating; a determined quality
level of a received signal; a direction of interference in a
communicated signal; a direction of transmission or convergence
speed associated with linearizing the output of the array; and a
direction of transmission or convergence speed associated with
calibrating the transmitter array.
The apparatus may further comprise means for determining the
non-linear part of the beam by calculating any of: an adjacent
channel power; a total radiated adjacent channel power; a signal to
noise ratio; and a signal to interference ratio.
The means for modifying the nonlinear characteristics between the
transmit paths may be configured to modify the nonlinear
characteristics by least one of: altering the phase of a phase
shifter in at least one transmit path; altering the gain of a
variable gain amplifier in at least one transmit path; and biasing
of a power amplifier in at least one transmit path.
According to a second aspect, there is provided a method
comprising: applying beam shape coefficients for obtaining a beam
shape output for a linear part of the beam formed by an array of
transmitters, each transmitter being associated with a respective
transmit path comprising a respective power amplifier; varying the
output characteristics of at least one transmit path to increase
nonlinear characteristics between the transmit paths to provide a
beam shape for a non-linear part of the beam; linearizing the
output of the array by: determining linearization coefficients for
the array for digitally predistorting the input signal to the
array; and applying the linearization coefficients to the signal
input to the array; and calibrating the transmitter array by:
measuring error distributions over at least two transmit paths;
modifying the beam shape coefficients to better obtain the beam
shapes subsequent to the linearization; and applying the modified
beam shape coefficients to the array of transmitters.
The method may further comprise alternating between the linearizing
and the calibrating until the beam shape, having a mixture of
linear and non-linear components, is obtained.
Measuring the error distribution over at least two transmit paths
may comprise measuring any of: a power distribution, an amplitude
distribution, and/or an envelope distribution over the at least two
transmit paths.
Determining the linearization coefficients may comprise obtaining
time domain samples of the signals output by the array.
The method may further comprise: comparing the measured error
distributions to a threshold value; and alternating between said
linearizing and said calibrating while the measured error
distributions are above the threshold value.
The method may further comprise: comparing the measured error
distributions to a threshold value; and operating the calibrating
when the measured error distributions are above the threshold
value; and operating the linearizing when the measured error
distributions are below the threshold value.
The method may further comprise: providing the linearization
coefficients to an apparatus performing the calibrating; and using
the linearization coefficients for modifying the beam shape
coefficients.
The method may further comprise altering the beam shape during the
operation of the apparatus based on at least one of: differences in
nonlinearities between nonlinear transmit paths; interference level
surrounding the apparatus; received signal power levels; movement
of the apparatus relative to other apparatuses with which the
apparatus is communicating; a determined quality level of a
received signal; a direction of interference in a communicated
signal; a direction of transmission or convergence speed associated
with linearizing the output of the array; and a direction of
transmission or convergence speed associated with calibrating the
transmitter array.
The method may further comprise determining the non-linear part of
the beam by calculating any of: an adjacent channel power; a total
radiated adjacent channel power; a signal to noise ratio; and a
signal to interference ratio.
The modifying the nonlinear characteristics between the transmit
paths may comprise modifying the nonlinear characteristics by least
one of: altering the phase of a phase shifter in at least one
transmit path; altering the gain of a variable gain amplifier in at
least one transmit path; and biasing of a power amplifier in at
least one transmit path.
According to a third aspect there is provided a computer program
comprising instructions for causing an apparatus to perform at
least: applying beam shape coefficients for obtaining a beam shape
output for a linear part of the beam formed by an array of
transmitters, each transmitter being associated with a respective
transmit path comprising a respective power amplifier; varying the
output characteristics of at least one transmit path to increase
nonlinear characteristics between the transmit paths to provide a
beam shape for a non-linear part of the beam; linearizing the
output of the array by: determining linearization coefficients for
the array for digitally predistorting the input signal to the
array; and applying the linearization coefficients to the signal
input to the array; and calibrating the transmitter array by:
measuring error distributions over at least two transmit paths;
modifying the beam shape coefficients to better obtain the beam
shapes subsequent to the linearization; and applying the modified
beam shape coefficients to the array of transmitters.
The instructions may further cause the apparatus to perform
alternating between the linearizing and the calibrating until the
beam shape, having a mixture of linear and non-linear components,
is obtained.
Measuring the error distribution over at least two transmit paths
may comprise measuring any of: a power distribution, an amplitude
distribution, and/or an envelope distribution over the at least two
transmit paths.
Determining the linearization coefficients may comprise obtaining
time domain samples of the signals output by the array.
The instructions may further cause the apparatus to perform:
comparing the measured error distributions to a threshold value;
and alternating between said linearizing and said calibrating while
the measured error distributions are above the threshold value.
The instructions may further cause the apparatus to perform:
comparing the measured error distributions to a threshold value;
and operating the calibrating when the measured error distributions
are above the threshold value; and operating the linearizing when
the measured error distributions are below the threshold value.
The instructions may further cause the apparatus to perform:
providing the linearization coefficients to an apparatus performing
the calibrating; and using the linearization coefficients for
modifying the beam shape coefficients.
The instructions may further cause the apparatus to perform
altering the beam shape during the operation of the apparatus based
on at least one of: differences in nonlinearities between nonlinear
transmit paths; interference level surrounding the apparatus;
received signal power levels; movement of the apparatus relative to
other apparatuses with which the apparatus is communicating; a
determined quality level of a received signal; a direction of
interference in a communicated signal; a direction of transmission
or convergence speed associated with linearizing the output of the
array; and a direction of transmission or convergence speed
associated with calibrating the transmitter array.
The instructions may further cause the apparatus to perform
determining the non-linear part of the beam by calculating any of:
an adjacent channel power; a total radiated adjacent channel power;
a signal to noise ratio; and a signal to interference ratio.
The modifying the nonlinear characteristics between the transmit
paths may comprise modifying the nonlinear characteristics by least
one of: altering the phase of a phase shifter in at least one
transmit path; altering the gain of a variable gain amplifier in at
least one transmit path; and biasing of a power amplifier in at
least one transmit path.
According to a fifth aspect there is provided a non-transitory
computer readable medium comprising program instructions for
causing an apparatus to perform at least the following: apply beam
shape coefficients for obtaining a beam shape output for a linear
part of the beam formed by an array of transmitters, each
transmitter being associated with a respective transmit path
comprising a respective power amplifier; vary the output
characteristics of at least one transmit path to increase nonlinear
characteristics between the transmit paths to provide a beam shape
for a non-linear part of the beam; linearize the output of the
array by: determining linearization coefficients for the array for
digitally predistorting the input signal to the array; and applying
the linearization coefficients to the signal input to the array;
and calibrate the transmitter array by: measuring error
distributions over at least two transmit paths; modifying the beam
shape coefficients to better obtain the beam shapes subsequent to
the linearization; and applying the modified beam shape
coefficients to the array of transmitters.
In an example, the non-transitory computer readable medium may
comprise instructions for causing an apparatus to perform at least
any of the steps of the examples described between the
above-mentioned second aspect.
According to a sixth aspect, there is provided an apparatus
comprising at least one processor, and at least one memory
including computer program code, wherein the at least one memory
and the computer program code are configured, with the at least one
processor, to cause the apparatus to: apply beam shape coefficients
for obtaining a beam shape output for a linear part of the beam
formed by an array of transmitters, each transmitter being
associated with a respective transmit path comprising a respective
power amplifier; vary the output characteristics of at least one
transmit path to increase nonlinear characteristics between the
transmit paths to provide a beam shape for a non-linear part of the
beam; linearize the output of the array by: determining
linearization coefficients for the array for digitally
predistorting the input signal to the array; and applying the
linearization coefficients to the signal input to the array; and
calibrate the transmitter array by: measuring error distributions
over at least two transmit paths; modifying the beam shape
coefficients to better obtain the beam shapes subsequent to the
linearization; and applying the modified beam shape coefficients to
the array of transmitters.
The apparatus may be further caused to alternate between the
linearizing and the calibrating until the beam shape, having a
mixture of linear and non-linear components, is obtained.
Measuring the error distribution over at least two transmit paths
may comprise measuring any of: a power distribution, an amplitude
distribution, and/or an envelope distribution over the at least two
transmit paths.
Determining the linearization coefficients may comprise obtaining
time domain samples of the signals output by the array.
The apparatus may be further caused to: compare the measured error
distributions to a threshold value; and alternate between said
linearizing and said calibrating while the measured error
distributions are above the threshold value.
The apparatus may be further caused to: compare the measured error
distributions to a threshold value; and operate the calibrating
when the measured error distributions are above the threshold
value; and operate the linearizing when the measured error
distributions are below the threshold value.
The apparatus may be further caused to: provide the linearization
coefficients to an apparatus performing the calibrating; and use
the linearization coefficients for modifying the beam shape
coefficients.
The apparatus may be further caused to alter the beam shape during
the operation of the apparatus based on at least one of:
differences in nonlinearities between nonlinear transmit paths;
interference level surrounding the apparatus; received signal power
levels; movement of the apparatus relative to other apparatuses
with which the apparatus is communicating; a determined quality
level of a received signal; a direction of interference in a
communicated signal; a direction of transmission or convergence
speed associated with linearizing the output of the array; and a
direction of transmission or convergence speed associated with
calibrating the transmitter array.
The apparatus may be further caused to determine the non-linear
part of the beam by calculating any of: an adjacent channel power;
a total radiated adjacent channel power; a signal to noise ratio;
and a signal to interference ratio.
Modifying the nonlinear characteristics between the transmit paths
may comprise modifying the nonlinear characteristics by least one
of: altering the phase of a phase shifter in at least one transmit
path; altering the gain of a variable gain amplifier in at least
one transmit path; and biasing of a power amplifier in at least one
transmit path.
DESCRIPTION OF FIGURES
Embodiments will now be described, by way of example only, with
reference to the accompanying Figures in which:
FIG. 1 shows a schematic diagram of an example communication system
comprising a plurality of base stations and a plurality of
communication devices;
FIG. 2 shows a schematic diagram of an example mobile communication
device;
FIG. 3 shows a schematic diagram of an example network element;
FIG. 4 shows a block diagram of an example system where a single
linearizer is applied for a plurality of nonlinear transmit
branches;
FIG. 5A shows an simulated example of a tapered power distribution
over antenna inputs when linear and nonlinear power amplifiers are
used and when a common linearizer is applied for linearizing a
plurality of power amplifiers;
FIG. 5B shows an example of simulated beam shapes of the channel
power over the azimuth half plane when linear and nonlinear power
amplifiers are used and when common linearizer is and is not
applied for linearizing a plurality of power amplifiers;
FIG. 5C shows an example of simulated beam shapes of the channel
power and adjacent channel power over the azimuth half plane when
nonlinear power amplifiers are used and when common linearizer is
and is not applied for linearizing a plurality of power
amplifiers;
FIG. 6 shows a block diagram of an example system where beam
calibration and common linearization processes are connected to
each other;
FIG. 7 shows a flow chart of the combined calibration and
linearization process;
FIG. 8 shows a flow chart of the combined calibration and
linearization process in which the calibration and linearization
processes are combined;
FIG. 9A shows an example of convergence of the process described in
FIG. 8 with and without step 804 of FIG. 8;
FIG. 9B shows an example of the power distributions over antennas
when process described in FIG. 8, with and without step 804 of FIG.
8, is and is not used;
FIG. 10 shows a flow chart of combined calibration and
linearization process in which the calibration process is repeated
until convergence before applying linearization process to improve
the stability of the process.
FIG. 11A shows example convergences of the process described in
FIG. 10, both with and without step 1204 of FIG. 10 being
performed;
FIG. 11B shows an example of power distributions over antennas when
the process described in FIG. 10, both with and without step 1204
of FIG. 10, is applied;
FIG. 12 shows a block diagram of an example system where beam
calibration and common linearization processes are separated from
each other;
FIG. 13 shows a block diagram of an example system where beam
calibration and common linearization processes are connected to
each other and power amplifier characteristics are controlled
during beam calibration and common linearization processes;
FIG. 14A shows a block diagram of an example system where beam
calibration is performed for signal envelopes or average envelopes
and combined with a linearization process;
FIG. 14B shows a schematic drawing of an example circuitry for
determining an envelope distribution over the antennas;
FIG. 14C shows a schematic drawing of an example circuitry for
determining an envelope difference distribution over the
antennas;
FIG. 15 shows a schematic drawing of an example difference
circuitry for calculating calibration coefficients and/or error
distributions over at least one antenna;
FIG. 16A shows an example of the distorted signal spectrum and
frequency response of the adjacent channel power filter shown in
FIG. 16B.
FIG. 16B shows a block diagram of an example system where beam
calibration and linearization processes are combined such that the
total radiated adjacent channel power performance is guaranteed;
and
FIG. 16C shows another block diagram of an example system where the
beam calibration and linearization processes are combined such that
the total radiated ACP performance is guaranteed but only one
shared adjacent channel power filter is used.
DETAILED DESCRIPTION
The present application relates to controlling linear and nonlinear
characteristics of a transmitter array having a plurality of
nonlinear paths.
In particular, as a signal is passed through a plurality of power
amplifiers, the power amplifier outputs may be combined to form an
overall output beam shape. Each power amplifier may have a linear
mode of operation and a nonlinear mode of operation, and so the
output beam shape may comprise both linear and nonlinear parts. The
following proposes that the linear and nonlinear characteristics of
such an output beam may be controlled by using a combination of
both beam shaping and linearization techniques. In particular, the
linear part(s) of the beam may be controlled using a calibration
process whilst the non-linear part(s) of the beam may be controlled
using linearization techniques, such as digital predistortion.
Before explaining in detail the examples, certain general
principles of a wireless communication system and mobile
communication devices are briefly explained with reference to FIGS.
1 to 2 to assist in understanding the technology underlying the
described examples.
In a wireless communication system 100, such as that shown in FIG.
1, mobile communication devices or user apparatus (UE) 102, 104,
105 are provided wireless access via at least one base station or
similar wireless transmitting and/or receiving node or point. A
user can access the communication system by means of an appropriate
communication device or terminal. A communication device of a user
is often referred to as user equipment (UE) or as a user apparatus.
Throughout the following, these terms will be used interchangeably.
A communication device is provided with an appropriate signal
receiving and transmitting apparatus for enabling communications,
for example enabling access to a communication network or
communications directly with other users. The communication device
may access a carrier provided by a station or access point, and
transmit and/or receive communications on the carrier.
The communication system and associated devices typically operate
in accordance with a given standard or specification which sets out
what the various entities associated with the system are permitted
to do and how that should be achieved. Communication protocols
and/or parameters which shall be used for the connection are also
typically defined. One example of a communications system is UTRAN
(3G radio). An example of attempts to solve the problems associated
with the increased demands for capacity is an architecture that is
known as the long-term evolution (LTE) of the Universal Mobile
Telecommunications System (UMTS) radio-access technology. LTE is
being standardized by the 3rd Generation Partnership Project
(3GPP).
A base station is referred to as an eNodeB (eNB) in LTE, and may be
referred to more generally as simply a network apparatus or a
network access point. Base stations are typically controlled by at
least one appropriate controller apparatus, so as to enable
operation thereof and management of mobile communication devices in
communication with the base stations. The controller apparatus may
be located in a radio access network (e.g. wireless communication
system 100) or in a core network (CN) (not shown) and may be
implemented as one central apparatus or its functionality may be
distributed over several apparatus. The controller apparatus may be
part of the base station and/or provided by a separate entity such
as a Radio Network Controller. In FIG. 1 control apparatus 108 and
109 are shown to control the respective macro level base stations
106 and 107. In some systems, the control apparatus may
additionally or alternatively be provided in a radio network
controller.
LTE systems may however be considered to have a so-called "flat"
architecture, without the provision of RNCs; rather the (e)NB is in
communication with a system architecture evolution gateway (SAE-GW)
and a mobility management entity (MME), which entities may also be
pooled meaning that a plurality of these nodes may serve a
plurality (set) of (e)NBs. Each user apparatus is served by only
one MME and/or S-GW at a time and the (e) NB keeps track of current
association. SAE-GW is a "high-level" user plane core network
element in LTE, which may consist of the S-GW and the P-GW (serving
gateway and packet data network gateway, respectively). The
functionalities of the S-GW and P-GW are separated and they are not
required to be co-located.
In an LTE system, radio resource control (RRC) is defined to be a
sublayer of radio interface Layer 3 that exists in the control
plane only, and which provides information transfer service to the
non-access stratum (see 3GPP Technical Specification Group Services
and System Aspects 21.905). RRC is a protocol layer between a user
apparatus and an eNB, and is in charge of, for example, paging the
user apparatus when traffic comes, establishing/maintaining or
release of radio bearers (establishing an RRC connection between
user apparatus and eNB), user apparatus mobility, user apparatus
measurement configuration and user apparatus reporting
configuration, etc. RRC is responsible for controlling the
configuration of radio interface Layers 1 and 2.
In FIG. 1 base stations 106 and 107 are shown as connected to a
wider communications network 113 via gateway 112. A further gateway
function may be provided to connect to another network.
The smaller base stations 116, 118 and 120 may also be connected to
the network 113, for example by a separate gateway function and/or
via the controllers of the macro level stations. The base stations
116, 118 and 120 may be pico or femto level base stations or the
like. In the example, base stations 116 and 118 are connected via a
gateway 111 whilst station 120 connects via the controller
apparatus 108. In some embodiments, the smaller stations may not be
provided.
A possible mobile communication device will now be described in
more detail with reference to FIG. 2 showing a schematic, partially
sectioned view of a communication device 200. Such a communication
device is often referred to as user apparatus (UE) or terminal. An
appropriate mobile communication device may be provided by any
device capable of sending and receiving radio signals. Non-limiting
examples comprise a mobile station (MS) or mobile device such as a
mobile phone or what is known as a `smart phone`, a computer
provided with a wireless interface card or other wireless interface
facility (e.g., USB dongle), personal data assistant (PDA) or a
tablet provided with wireless communication capabilities, or any
combinations of these or the like. A mobile communication device
may provide, for example, communication of data for carrying
communications such as voice, electronic mail (email), text
message, multimedia and so on. Users may thus be offered and
provided numerous services via their communication devices.
Non-limiting examples of these services comprise two-way or
multi-way calls, data communication or multimedia services or
simply an access to a data communications network system, such as
the Internet. Users may also be provided broadcast or multicast
data. Non-limiting examples of the content comprise downloads,
television and radio programs, videos, advertisements, various
alerts and other information.
The mobile device 200 may receive signals over an air or radio
interface 207 via appropriate apparatus for receiving and may
transmit signals via appropriate apparatus for transmitting radio
signals. In FIG. 2 transceiver apparatus is designated
schematically by block 206 for transmitting and receiving signals
over the air or radio interface 207. The transceiver apparatus 206
may be provided for example by means of a radio part and associated
antenna arrangement. The antenna arrangement may be a single
antenna or an antenna array. The antenna arrangement may be
arranged internally or externally to the mobile device.
A mobile device is typically provided with at least one data
processing entity 201, at least one memory 202 and other possible
components 203 for use in software and hardware aided execution of
tasks it is designed to perform, including control of access to and
communications with access systems and other communication devices.
The data processing, storage and other relevant control apparatus
can be provided on an appropriate circuit board and/or in chipsets.
This feature is denoted by reference 204. The user may control the
operation of the mobile device by means of a suitable user
interface such as key pad 205, voice commands, touch sensitive
screen or pad, combinations thereof or the like. A display 208, a
speaker and a microphone can be also provided. Furthermore, a
mobile communication device may comprise appropriate connectors
(either wired or wireless) to other devices and/or for connecting
external accessories, for example hands-free equipment, thereto.
The communication devices 102, 104, 105 may access the
communication system based on various access techniques.
An example of wireless communication systems are architectures
standardized by the 3rd Generation Partnership Project (3GPP). A
latest 3GPP based development is often referred to as the long term
evolution (LTE) or LTE Advanced Pro of the Universal Mobile
Telecommunications System (UMTS) radio-access technology. Other
examples of radio access system comprise those provided by base
stations of systems that are based on technologies such as wireless
local area network (WLAN) and/or WiMax (Worldwide Interoperability
for Microwave Access). A base station can provide coverage for an
entire cell or similar radio service area.
An example network equipment for the 3GPP system is shown in FIG.
3. FIG. 3 shows an example of a control apparatus 300 for a
communication system, for example to be coupled to and/or for
controlling a station of an access system, such as a RAN node, e.g.
a base station or (e) node B, or a node of a core network such as
an MME. In FIG. 1, these access nodes may be any of 106, 107, 116,
118, 120. The method may be implanted in a single control apparatus
or across more than one control apparatus. The control apparatus
may be integrated with or external to a node or module of a core
network or RAN. In some embodiments, base stations comprise a
separate control apparatus unit or module. In other embodiments,
the control apparatus can be another network element such as a
radio network controller or a spectrum controller. In some
embodiments, each base station may have such a control apparatus as
well as a control apparatus being provided in a radio network
controller. The control apparatus 300 can be arranged to provide
control on communications in the service area of the system. The
control apparatus 300 comprises at least one memory 301, at least
one data processing unit 302, 303 and an input/output interface
304. Via the interface, the control apparatus can be coupled to a
receiver and a transmitter of the base station. The receiver and/or
the transmitter may be implemented as a radio front end or a remote
radio head, and may function in a similar manner to transceiver
apparatus 206.
For example the control apparatus 300 can be configured to execute
an appropriate software code to provide the control functions.
Control apparatus 300 may be included in a chipset or modem
apparatus. A chipset or modem apparatus which includes apparatus
300 may be included in a control node such as an eNB (Radio base
station).
Communication devices may utilise arrays of antennas when
communicating between devices. For example, a transmitter may have
an array of transmitter antennas for transmitting a signal to a
receiver, whilst a receiver may have an array of receiver antennas
for receiving a transmitted signal. The following will focus on
transmitter arrays.
A transmitter array of a device is a set of two or more
transmitters with respective non-linear signal branches, where the
signals from the signal branches of the transmitters are combined
or processed to achieve improved performance for the device's
transmissions. Each signal path (also referred to herein as a
transmitter path) comprises at least one respective power amplifier
for increasing the transmission power of a signal being processed
on that signal path and a respective antenna for transmitting the
signal processed along the signal path. The signal may be
transmitted over the air (for example, at radio frequencies or at
microwave frequencies). A transmitter array can be used to increase
the overall transmission gain, to "steer" a transmission beam into
a particular direction, "shape" it to have a certain spatial shape,
and/or to maximise a Signal to Interference plus Noise Ratio
(SINR).
For steering a beam in a particular direction, phased transmitter
arrays may be used. Phased transmitter arrays are transmitter
arrays in which the direction of a transmitted beam may be steered
electronically, without physically moving the antennas themselves.
Instead, a signal is passed to multiple signal branches, each
signal branch comprising a phase shifter that electronically alters
the phase of the signal in that signal branch. This can be used to
direct/steer a beam of radio waves in a particular direction. In
addition, the gain of each phased array branch may be controlled
e.g. by variable gain amplifiers such that the beam shape can be
also varied. In other words, the amplitudes of each phased array
branch may be controlled. To practically achieve this, a large
number of small antennas are commonly used (sometimes of the orders
of thousands of antennas) in order to achieve high gain, phased
arrays.
Phased array transmitters are becoming popular for millimetre-wave
(mmW) band communications and are the emerging technology towards
5G communication networks. Phased antenna transceivers under 6 GHz
frequency range are also emerging as a popular technology for 5G
communication network. As 5G networks may require a high data rate
of transmissions, this means that the phased arrays should operate
in a linear mode to guarantee high-quality communications.
Linearity commonly refers to a property of a power amplifier. In
particular, a power amplifier exhibits two operation regions. In a
first region, a power amplifier presents linear behaviour (i.e. the
output power is proportional to the input power) but operates with
a low efficiency (i.e. a low ratio between the output power and the
total power consumption). In a second region, a power amplifier
presents nonlinear behaviour (i.e. the output power is not
proportional to the input power) but operates with a high
efficiency (i.e. a high ratio between the output power and the
total power consumption). Although a highly linear, highly
efficient signal is desired, constraints in power amplifier design
may make this difficult to achieve.
One mechanism for obtaining a highly linear, highly efficient
signal is to distort the peaks of the communication signals to
achieve high efficiency and to compensate later for the non-linear
distortion introduced by, for example, predistortion linearization.
Predistortion linearization, including digital predistortion,
compensates for nonlinear behaviour of power amplifiers by
modifying the phase and/or amplitude of each sample of a power
amplifier input signal. The predistortion shapes the signal over
time or frequency domain samples. This signal modification is
performed by analysing the output signal of the power amplifier and
generating a complementary nonlinearity to that of the power
amplifier to even it out.
The following describes techniques for linearizing a signal that
were disclosed in earlier filed application GB 1720236.7, with
reference to FIGS. 4 to 5.
FIG. 4 illustrates an input waveform being input into a digital
predistorter 401. The digital predistorter 401 distorts the input
signal based on predistortion coefficients h (discussed later). The
digital predistorter 401 outputs the distorted waveform to a
digital to analogue converter 402, which converts the signal into
analogue to a local oscillator 403. The local oscillator 403
upconverts the signal to a higher frequency and outputs the
upconverted signal to a plurality of signal branches 404a, 404b . .
. 404n. Each signal branch is non-linear and comprises respective
phase shifters 405a, 405b . . . 405n (which apply respective phase
shifts in order to control the beam direction), variable gain
amplifiers 406a, 406b . . . 406n, power amplifiers 407a, 407b . . .
407n, coupling elements 408a, 408b . . . 408n, which provides a
copy of the signal in a branch to a respective antenna 409a, 409b .
. . 409n and to feedback circuitry comprising a phase shifter 410a,
410b . . . 410n (which may apply a negative phase shift to that
applied by phase shifters 405a, 405b . . . 405n) and a controllable
attenuator 411a, 411b . . . 411n to attenuate the signal before a
summation. The sum of the outputs from all of these branches is
input to a local oscillator 412, which downconverts the summed
signal to a lower frequency. The downconverted signal is then input
to an analogue to digital converter 413, which converts the
downconvered signal into digital representation at a sampling
frequency. The digital representation/digital domain samples are
then input to a digital predistortion calculator 414, which
determines the predistortion coefficients h to be used by the
digital predistorter 401 in distorting the input signal.
Thus, the system illustrated with respect to FIG. 4 applies a
single set of digital predistortion coefficients for an array of
signal branches. The coefficients are selected such that the
individual power amplifier outputs result in a linear combination
of nonlinear power amplifier outputs when combined over the air.
For example, the modelling and/or measuring of the combination of
nonlinearities in the beamforming direction may be performed by
utilising an array factor (e.g. a factor by which the directivity
function of an individual antenna can be multiplied to get the
directivity of the entire array), where the digital predistortion
coefficients are based on the combined feedback output.
The phased-array transmitter may control a transmission power of
the transmitter either during production, calibration and/or
operation of the apparatus. One of the most common approaches is to
use the variable gain amplifiers 406a, 406b . . . 406n for this
purpose. A power control algorithm controls the variable gain
amplifiers' 406a, 406b . . . 406n operation to produce needed
predetermined transmission power. Thus, the power control algorithm
may control individual gains of the variable gain amplifiers 406a,
406b . . . 406n to comply with a combined transmission power
requirement. The power control algorithm may be combined with the
digital predistortion algorithm to optimize digital predistortion
performance.
The phased-array transmitter may also use the variable gain
amplifiers to shape the beam, e.g. in order to create nulls and/or
maxima to a desired direction or directions. The shaping may be
desired, for example, in order to decrease interference in some
direction or directions. Many beamforming techniques such as
zero-forcing (ZF) or maximum ratio transmission (MRT) control the
amplitude of each antenna branch/transmit path in addition to the
phase of each branch. The gain control may also be used to design
the signal formed by the array output to have a certain sidelobe
level, i.e. to decrease the interference in all other directions
compared with the main beam. Such techniques are referred as
amplitude tapering methods. An example amplitude tapering method is
Taylor amplitude tapering.
The overall output from the array (aka the array behavior) can be
measured and/or modelled by sampling the power amplifier outputs
one by one and combining their responses. The measuring and/or
modelling of the behavior may be done by utilizing a feedback
structure such as the one depicted in FIG. 4, which "mimics" the
array behavior of the power amplifier outputs in RF domain.
The input powers of individual power amplifiers 407a, 407b . . .
407n may be varied to control of the variable gain amplifiers 406a,
406b . . . 406n such that the power amplifiers 407a, 407b . . .
407n are operating in different linearity modes. Thus, the
introduced predistortion from the predistortion coefficients may
cause expansion for some of the power amplifiers 407a, 407b . . .
407n, which can compensate the compression of others of the power
amplifiers 407a, 407b . . . 407n when the compressive-expansive
behavior is combined over the air. The combining can be emulated by
the set of controllable phase shifters 410a, 410b . . . 410n and
the controllable attenuators in the feedback branch(s). Thus, the
phase shifters 410a, 410b . . . 410n and the controllable
attenuators 411a, 411b . . . 411n in the feedback branch emulate
the radio channel, while those signals combined over the air are
finally received at the receiver.
FIG. 5A shows an amplitude distribution over power amplifier
outputs with Taylor tapering (target side lobe level is set to 30
dBc). The "ideal" amplitude distribution with "ideal" linear power
amplifiers is shown at the top curve. The middle curve represents
the amplitude distribution with nonlinear power amplifiers and
without digital predistortion whilst the bottom curve represents
the amplitude distribution with nonlinear power amplifiers and with
array digital predistortion. Compared to the "ideal" distribution,
the curve with the digital predistortion applied is compressed more
towards the peak than the curve without digital predistortion.
However, towards the tails, the curve with the digital
predistortion applied has expanded more than both the "ideal" curve
and the curve without digital predistortion.
Output powers from centrally located antennas are compressed when
nonlinear power amplifiers are used without array digital
predistortion. This is shown through the difference between the
middle curve and the top curve. Output powers from the outermost
antennas remain the same both with and without nonlinear power
amplifiers. In contrast, when predistortion techniques are applied
(bottom curve), the output powers from the central antennas are
compressed more compared to the case without predistortion
techniques being applied (middle curve) and output powers from the
outer edge antenna are expanded because of the digital
predistortion. Hence, any nonlinear regions are compressed more for
the higher driven power amplifiers than for the lower driven power
amplifiers. Moreover, digital predistortion expands the outermost
power amplifiers and central power amplifiers by different amounts,
which means that the digital predistortion has also changed the
power distribution over the antennas. This results in changes in
the beam shape compared with the case without linearization, as
shown in FIG. 5B.
FIG. 5B shows beam patterns of antenna array in three difference
scenarios: (1) "Ideal" pattern with linear power amplifiers are
used, (2) "w/o DPD" pattern when nonlinear power amplifiers without
digital predistortion are used, and (3) "w Array DPD" when
nonlinear power amplifiers with digital predistortion are used. The
plots for the arrays with digital predistortion show a compressed
main lobe/beam and larger sidelobes immediately adjacent to the
main beam. FIG. 5B shows that digital predistortion cannot recover
the beam shape as the present digital predistortion technique is
common for the array of parallel nonlinear power amplifiers. FIGS.
5A and 5B together illustrate that beam shape and digital
predistortion have a relation to each other.
FIG. 5C shows beam patterns of a channel power measured over the
main channel of a communication signal and adjacent channel power
measured over adjacent channels of a communication signal output by
an array of nonlinear power amplifiers. The solid plot having a
main lobe coincident with the 30.degree. direction presents the
maximum of lower and upper adjacent channel power (ACP) without
digital predistortion, and the solid plot having a minima along the
30.degree. direction gives the respective curve for the case in
which digital predistortion is applied. The dashed lines presents
the beam pattern of the channel power in the same manner as those
depicted in FIG. 5B. The ACP beam with digital predistortion makes
a notch in the beamforming direction because a common digital
predistortion is calculated in that direction. In other directions,
the ACP is slightly increased compared with the case without
digital predistortion. Similar to this, signal quality is maximized
in the beamforming direction by the digital predistortion. Thus
FIG. 5C shows that when a common linearizer is applied for a
plurality of nonlinear power amplifiers, the nonlinear part and
linear part of the signal may have a beam shape different from each
other.
As shown by example in FIG. 5C, utilising the array linearization
as per the system described with respect to FIG. 4 results in
over-the-air linearization in which the beam shape of the nonlinear
part of the signal may differ from the beam shape of the linear
part of the signal. However, as shown in FIGS. 5A and 5B, the array
predistortion changes the beam shape. In particular, if the
amplitudes, bias points of the power amplifiers, operational
voltages of the power amplifiers and/or mutual couplings between
individual elements are varied, the beam shape will also vary with
the input amplitudes and phases, leading to the beam pattern
becoming signal envelope-dependent. Envelope-dependent beam means
that each time or frequency domain waveform sample measured over
the spatial domain in different directions may draw different beam
shape due to the nonlinearity. However, amplitude differences
between power amplifiers are used to shape the beam of the linear
and nonlinear parts, and so introducing additional differences
between the power amplifiers for linearization purposes will result
in errors in the radiated beam shape, despite the differences
enabling better linearization results (as shown in the example
described with respect to FIG. 5C).
It is also useful to employ a calibration scheme to determine the
power amplifier operation points accurately in order to both
maximise efficiency of the power amplifiers and to stabilize the
digital predistortion algorithm used. However, if power amplifiers
have differences in their operation points, the beam calibration
will affect the linearization, and vice-versa. In other words,
changing power amplifier responses in either a linear or a
non-linear way can change the overall linearity of the system over
the air in a certain direction or a set of directions in which the
array is intended to be linearized. In particular, the
linearization may make individual amplifiers expand or compress
their output signal (depending on their nonlinear characteristics),
which will also change the beam shape. In other words, there is a
symbiotic relationship between linearization and calibration
operations. Therefore, to obtain both an intended beam shape and
linearity (efficiency) simultaneously, the following proposes
techniques for combining linearization and calibration
operations.
FIG. 6 illustrates a potential system in which this technique may
be employed.
FIG. 6 illustrates an input waveform being input from a processing
unit 600 into a digital predistorter 601, which is coming from
processing unit 620. The digital predistorter 601 distorts the
input signal based on predistortion coefficients h (discussed
later), which may be determining applying least squares fitting to
solve the predistortion coefficients. The mathematical model may
be, for example, a polynomial, memory polynomial or an envelope
polynomial. The predistortion coefficients are coefficients of the
mathematical model, which tries to model non-linear behaviour of
the radio system. Other approaches for calculating predistortion
coefficients h can be applied as well. In adaptive array DPD, the
coefficients may be solved also by adaptive filtering methods such
as least mean squares (LMS), recursive least squares (RLS),
Kalman-filtering, etc. The digital predistorter 601 outputs the
distorted waveform to a digital to analogue converter 602, which
converts the signal into analogue form to a local oscillator 603.
The local oscillator 603 upconverts the signal to a higher
frequency and outputs the upconverted signal to a plurality of
signal branches 604a, 604b . . . 604n. Each signal branch is
non-linear and comprises respective phase shifters 605a, 605b . . .
605n (which apply respective phase shifts), variable gain
amplifiers 606a, 606b . . . 606n, power amplifiers 607a, 607b . . .
607n, coupling elements 608a, 608b . . . 608n, which provides a
copy of the signal in a branch to a respective antenna 609a, 609b .
. . 609n and to feedback circuitry comprising a phase shifter 610a,
610b . . . 610n (which apply an inverse phase shift to that applied
by phase shifters 605a, 605b . . . 605n) and a controllable
attenuator 611a, 611b . . . 611n. The sum of the output from all of
these branches is output to a local oscillator 612, which
downconverts the summed signal to a lower frequency. The
downconverted signal is then output to an analogue to digital
converter 613, which converts the downconverted signal into digital
representation by sampling at a sampling frequency. The digital
representation/digital domain samples of the signal are input to a
digital predistortion calculator 614, which determines the
predistortion coefficients h to be used by the digital predistorter
601 in distorting the input signal.
Thus, the system illustrated with respect to FIG. 6, like that of
FIG. 4, applies a single set of digital predistortion coefficients
for an array of signal branches. The coefficients are selected such
that the individual power amplifier outputs result in a linear
combination of nonlinear power amplifier outputs when combined over
the air. For example, the modelling and/or measuring of the
combination of nonlinearities in the beamforming direction may be
performed by utilising an array factor (e.g. a factor by which the
directivity function of an individual antenna can be multiplied to
get the directivity of the entire array), where the digital
predistortion coefficients are based on the combined feedback
output. This feedback output may be as described in relation to
FIG. 4.
In addition to those components also displayed in FIG. 4, FIG. 6
also comprises a beam calibration mechanism. This is represented in
FIG. 6 by target beamformer 615, a beam calibration unit 616 and a
measurement unit 617 (represented by a single box taking input from
the output of each of coupling elements 608a, 608b . . . 608n). For
example, if the power of each amplifier is used for calibration,
the measurement unit may include power detectors for measuring the
respective output power of each power amplifier input. Alternative
measurement mechanisms may be used for performing calibration, such
as described below.
In operation, the target beamformer 615 provides information on an
intended beam shape to the calibration unit 616. The measurement
unit 617 provides feedback information to the calibration unit 616
on the signal being passed for transmission by the antennas. This
feedback information may be on at least one of the following
factors: the powers output by the power amplifiers, the voltages
applied across each power amplifier, the signal envelopes of each
individual branch, and the average envelopes of the individual
branches. The feedback information may provide relate to a
distribution over at least one antenna (or may relate to a
distribution over a plurality of antennas in an antenna array).
This feedback information thus essentially indicates the current
output beam shape. The calibration unit may use this feedback
information, in addition to the provided intended beam shape, to
produce calibration coefficients for controlling each of the
variable gain amplifiers 606a, 606b . . . 606n and/or phase
shifters 607a, 607b, . . . , 607n.
A flow chart detailing a potential operation of a system such as
that illustrated with respect to FIG. 6 is provided in FIG. 7. FIG.
7 relates to a combined calibration and linearization process in
which the power amplifier characteristics are pre-varied to
increase the variability of nonlinearities of the nonlinear
transmit paths and in which the power amplifier characteristics are
also calibrated to form a desired (target) beam shape.
At 701, first beam shape coefficients for obtaining a first beam
shape output for a linear part of a beam formed by an array of
transmitters are applied by the array of transmitters. The first
beam shape may be saved in a memory unit. This memory unit may
store the first beam shape in a variety of different ways. For
example, the first beam shape may be saved as register values,
target bias voltages, currents and/or target output voltages,
powers, signal envelopes and/or average envelopes. The first beam
shape may include intended gain/power/phase variations over
transmitter branches. Each transmitter is associated with a
respective transmit path comprising a respective power amplifier.
The first beam shape may be formed from target beamformer values
taken from a memory of a processing unit of the apparatus. Beam
shaping, in general, means that the shape of the overall beam may
be controlled by controlling the amplitudes and phases of
individual paths by, for example, controlling the phases shifts,
delays, power gains and/or bias currents of the individual
branches. Thus changes to an amplitude balance over all of the
antennae/power amplifiers can result in changes to the beam shape.
After the first beam shape has passed through the power amplifier,
the non-linear parts of the power amplifiers causes the amplitude
and phase distribution of the output signal to differ from the
amplitude and phase distribution of the input signal. The
processing unit may alter the target beam shape during the
operation of the apparatus based on at least one of: the
differences in the nonlinearity between the nonlinear branches, the
interference level surrounding the transmitting radio apparatus,
received signal power levels, movement of the mobile devices,
quality level of the received signal, the direction of
interference, direction of transmission.
At 702, the characteristics of at least one transmit path are
varied to increase nonlinear characteristics between the transmit
paths. The variation is performed to provide a target beam shape
for a non-linear part of the beam formed by the array of
transmitters. In other words, the characteristics of the at least
one transmit path are varied to increase the variability between
non-linearity of transmit paths. As an example, the characteristics
may be varied in a number of different ways. Examples of how this
variation can be effected is via at least one of: altering the
phase of a phase shifter in at least one transmit path, by altering
the gain of a variable gain amplifier in at least one transmit
path, and biasing of a power amplifier in at least one transmit
path. The point of these operations is to give different signal
paths different nonlinearities to each other in order that the
distortion beam shape can be shaped differently to the linear part
of the output beam shape.
At 703, a linearization is applied to linearize the output of the
array of transmitters. In this sense, it is understood that
linearization is a mathematical process of finding a linear
approximation of inputs and corresponding outputs. This
linearization may be performed by determining first linearization
coefficients for the array for digitally predistorting the input
signal to the array and applying the first linearization
coefficients to the signal input to the array. For example, the
power amplifier array response may be measured using a feedback
receiver that mimics or otherwise emulates the radio channel in
some specific spatial direction. The predistortion may be performed
for the waveform (which comprises time domain samples). Information
from this feedback receiver may then be used to provide
linearization coefficients for the array (digital predistortion).
In principle, as discussed above, the linearization output has an
expansive shape, which means that some power amplifiers are
expanding their input signal for output whilst others are
contracting their input signal for output. This means that the beam
of the distortion is also being shaped by the linearization, which
affects each individual branch differently depending on their
characteristics. This, in turn, changes both the nonlinear and
linear parts of the beam. To correct this, the calibration
coefficients may be re-calculated with the initial calibration
coefficients in 701.
At 704, a calibration is applied to calibrate the powers of the
power amplifiers outputs of the transmitter array. In this sense,
it is understood that calibration is an electrical and/or a
mathematical process for configuring an instrument to provide an
output for an input within an acceptable range. This calibration
may be performed by measuring error distributions over at least two
transmit paths, modifying the first beam shape coefficients to
better obtain the first beam shape subsequent to the linearization,
and applying the modified first beam shape coefficients to the
array of transmitters. In this sense, the term "better" is used to
indicate that the signal output by the transmitter array more
closely resembles the target beam shape by applying the modified
first beam shape coefficients than if the first beam shape
coefficients were unmodified. The amplitude or power distribution
over the antennas may be measured using the powers of or envelope
functions of the individual branches. As mentioned above, either of
these distributions may be used to determine the actual output beam
shape, which affects both the linear and non-linear parts of the
beam. The calibration scheme may be performed over the antennas
(power, amplitude or envelope distribution). The point of the
calibration process is to control the shape of the linear part of
the beam while 702 and 703 controls the shape of the nonlinear part
of the beam.
The process may alternate between the calibrating/beam shaping
process and the common linearization operational modes until a
desire beam shape of linear and non-linear components is achieved.
It is useful to provide several iterations if linear correction is
used as the array is nonlinear.
Thus, as described above, the presented phased array system has
both a common lineariser and calibration and, in contrast to the
previously described system, there is provided a beam calibration
part that is used to calibrate the nonlinear beam shape. The beam
calibration may tune/calibrate signal paths individually. This is
in contrast to the common linearization scheme in which a common
set of predistortion coefficients are used to shape the signal
input to the array.
FIG. 8 provides an algorithm description for beam calibration being
performed with linearization techniques.
The steps of FIG. 8 may be applied while a determined beam error is
larger than a threshold value. The threshold value may alter/be
varied during the operation of the apparatus performing the steps
of FIG. 8.
At 801, an output beam is determined by measuring the power
distribution of power amplifier outputs. It is understood that this
is only one mechanism for determining the output beamform and that
other mechanisms, such as those discussed herein may also or
alternatively be used to determine the output beam in 801.
At 802, beam correction coefficients are solved over all amplitudes
between the target beamformer values and the measured beamformer.
The target beamformer values may be obtained from a memory unit, as
per the above-described examples. The measured power distribution
may be obtained from the measured power amplifier outputs in the
present example.
At 803, the solved beam correction coefficients are applied to
alter the outputs of the power amplifiers. The beam correction
coefficients may cause a different correction factor to be applied
to different power amplifiers. In other words, each signal path is
individually/separately calibrated.
At 804, waveform digital predistortion is performed, as described
above.
At 805, the new beam error from the combination of the applied
waveform digital predistortion and the applied beam correction
coefficients is calculated. The process then repeats until the
calculated beam error is less than the threshold value.
FIGS. 9A and 9B show effects of the applied calibration.
FIG. 9A illustrates how the calibration error decreases as the
number of iterations of this mechanism discussed in FIG. 8
increases. The bottom plot on this graph shows calibration
convergence without digital predistortion while the upper plot
shows calibration convergence with digital predistortion.
FIG. 9B shows how the target beam shape alters in dependence on the
process applied. In particular, the bottom curve (red) shows how
the target beam is compressed when neither calibration nor digital
predistortion is applied. In contrast the beam formed when beam
calibration or beam calibration and array digital predistortion is
used is substantially similar to the ideal line, and overlap each
other. Hence, the mechanism discussed in FIG. 8 calibrates the
output power distribution over the array when digital predistortion
is also applied.
The algorithm described with respect to FIG. 8 may occasionally
develop stability issues if the array is very nonlinear. This is
because calibration changes the operation point in which the
digital predistortion is operating and digital predistortion
changes the operation point in which the calibration is operating.
In order to address this, an alternative version of the algorithm
may be used. This is described below in relation to FIGS. 10 and
11.
In FIG. 10, while the beam error is larger than a threshold value,
the system applies steps 1201 to 1203.
At 1201, the output beam is measured (e.g. using the power
distribution of the power amplifiers).
At 1202, beam correction coefficients are determined over
amplitudes between the target beamformer and the measured
beamformer.
At 1203, the beam correction coefficients are applied to the
transmission paths and a new beam error is calculated. The process
then starts again from 1201 until the calculated beam error is
smaller than the threshold value. When the calculated beam error is
smaller than the threshold value, steps 1204 and 1205 are applied.
The actual threshold values used may be determined with respect to
the actual system them are being applied in and any threshold
values may be varied during the operation.
At 1204, array predistortion is performed.
At 1205, the beam error from the digital predistortion is
determined. If this determined beam error is less than the
threshold value, the process goes back to steps 1202 in order to
calculate new calibration coefficients for input signal being
predistorted by the coefficients derived in step 1204. The
threshold value used in step 1205 may be different from the
threshold value used in the step 1201 in order to improve the
algorithm convergence.
Thus, FIG. 10 relates to a system in which calibration is performed
first until a beam error below a certain value is obtained, before
linearization is performed. Linearization is then applied and the
process is continued until the beam error from linearization is
smaller than a threshold value. The system may switch between the
two operations (linearization and calibration) if the determined
beam errors cause them to (i.e. if the comparison of the measured
error to the respective threshold causes this switch to
happen).
FIGS. 11A and 11B illustrate the effect of calibration.
FIG. 11A illustrates how the calibration error decreases as the
number of iterations of this mechanism increases. The calibration
convergence without digital predistortion doesn't initially drop to
as low a calibration error as the calibration convergence with
digital predistortion plot, but exhibits an intermediate zone in
which calibration convergence lower than that of the calibration
convergence with digital predistortion plot.
FIG. 11B shows how the target distribution of the antenna inputs
changes in dependence on the process applied. In particular, the
bottom curve shows that the target distribution is compressed when
neither calibration nor digital predistortion is applied. In
contrast the beam formed when beam calibration is used is
substantially similar to the ideal line, whilst the curve for beam
calibration and array digital predistortion is slightly below the
ideal line. The reason for differences between the curves with and
without digital predistortion and the ideal line is the used
threshold values in simulations. The curves can converge to each
other with appropriate threshold value selection. The actual shape
of the curve is dependent on the threshold values applied, and so a
system designer may tailor this to the actual equipment being used
in a particular environment. Any changes in the amplitude
distribution over the antenna elements results in changes in the
intended beam shape.
FIG. 12 shows an example block diagram in which the array
calibration process and linearization are further separated from
each other. This may be performed, for example, if the calibration
process is significantly faster than the digital predistortion
process. The flows of the processes are similar to those shown in
FIG. 10, but differs in that information is not shared between the
calibration and linearization processes. In such a case, the array
calibration may be automatic in that the digital predistortion
process automatically takes into account the changes in the
calibration coefficients. This may be effected by two loops running
in parallel, as described further below. Elements are shown having
the same numbering as those used in FIG. 6.
In the first loop 1401, array calibration is performed and
comprises target beamformer 615, a calibration unit 616 and
measurement unit 617 (represented by a single box taking input from
the output of each of coupling elements 608a, 608b . . . 608n). The
first loop may perform calibration in three steps. In the first
step, the actual beam shape is determined (e.g. using the power
distribution output by the power amplifiers or some other
measurement, such as described above). In the second step, beam
coefficients are determined by comparing differences between the
target beamformer and the measured beamformer. In the third step,
the amplitudes of the power amplifiers are varied using the
variable gain amplifiers to attempt to remove the compared
differences.
In the second loop 1402, linearization (such as digital
predistortion) is performed. The second loop 1402 may comprise
phase shifters 610a, 610b . . . 610n, controllable attenuators
611a, 611b . . . 611n, local oscillator 612, analogue to digital
converter 613, digital predistortion calculator 614 and digital
predistorter 601. The second step may be performed in two steps. In
the first step, the combined nonlinearity of the output wave is
measured. In a second step, digital predistortion is performed to
predistort the array input waveform (as described above in
connection with FIG. 4).
FIG. 13 shows an example of an arrangement in which the beam
calibration unit may also control at least one of the bias
voltages, currents, gains, or operation points of individual power
amplifiers. The power amplifier characteristics may be controlled
in order to increase the efficiency of individual branches and/or
increasing the variability of nonlinear branches in order to
produce variations that can be further used to cancel each other
over the air by the digital predistortion. FIG. 13 uses the same
reference numerals as FIG. 6 to indicate similar elements.
FIG. 14A shows another example of how beam calibration and
linearization may be combined. FIG. 14a uses the same reference
numerals as FIG. 6 to indicate similar elements.
In this example, the calibration unit is arranged to follow the
output envelopes of each power amplifier or the average envelopes
of each power amplifier and to change the operational voltages of
individual power amplifiers to cause the power amplifiers to
operate in a more efficient mode. Alternatively or in addition, the
bias points of the power amplifiers may be changed based on the
measured envelopes. As the tracking may be performed as a function
of an envelope (i.e. on the function of an average amplitude of a
waveform), then the beam calibration also corrects the beam pattern
of the envelope.
Example implementation of measurement circuitry blocks for
measuring the signal envelope distribution over the antenna inputs
in FIG. 14A are illustrated with respect to FIGS. 14B and 14C.
In FIG. 14B, an arrangement for detecting the envelope or power
distribution over an array of antennas is provided as follows.
There is provided a plurality of transmit paths 1600a, 1600b, . . .
1600n, each transmit path providing a respective input 1601a,
1601b, . . . to a respective diode 1602a, 1602b, . . . 1602n. A
respective series of capacitors 1603a, 1603b, . . . 1603n and a
series of resistors 1604a, 1604b, . . . 1604n are connected in
parallel to each diode. A respective output 1605a, 1605b, . . .
1605n represents a detected envelope. If the detectors are
connected to each other, the envelope distribution can be detected
from this output, e.g. from the nodes Vel VeNa of the detection
circuit.
FIG. 14C provides another arrangement for detecting the envelope or
power distribution over an array of antennas. FIG. 14C utilises the
same labelling as FIG. 14B to denote similar element. This
arrangement differs from that of FIG. 14B in that the envelope
detection circuits are connected to each other. This circuitry thus
gives an envelope distribution with respect to an adjacent antenna
and hence the circuitry directly measures the envelope differences
over the antenna branches. This arrangement can be alternatively
connected to coupling elements placed after phase shifters in the
feedback circuitry of FIG. 14A, such that the phases of the inputs
to the envelope detectors VFB1 . . . VFBNa can be controlled.
An example implementation of difference circuitry of calibration
unit in FIG. 14A for producing the calibration weights based on the
target beamformer and measured envelope or power distribution is
shown in FIG. 15.
In FIG. 15, there is shown a plurality of "difference circuits",
each difference circuit being associated with a respective power
amplifier/signal path. For each difference circuit, a voltage
representing a target beamformer signal is input, a voltage signal
from the measurement unit (e.g. from power or envelope detectors)
associated with the power amplifier/signal path of that difference
circuit is input, and a new control/calibration voltage for the
associated power amplifier is output. The difference circuits may
comprise, for example, an operational amplifier and a plurality of
resistors. One resistor R1 may lie between an input of the
operational amplifier and the input of the target beamformer
values. Another resistor, R2, may be connected between the output
of R1 and the output of the difference circuit. R2 may further be
set towards ground from other operational amplifiers. Another
resistor, R3, may be connected between another input of the
operational amplifier and the voltage from the measurement
unit.
The target beamformer values are changed to analogue voltages by
voltage digital to analogue converters (VDACs). Each measured
envelope, average envelope or power is compared against the
analogue target beamformer voltages of the respective branch by the
difference circuit. If R3 is chosen to be very small and R1 is
chosen to be same value as R2, the new calibration coeffcients can
be directly read from the outputs of the differentiator circuits as
Vc.sub.1 . . . Vc.sub.N. Alternatively, if R3 is chosen to be same
as R1 and R2, the circuitry gives the difference between the input
voltages that can be further processed to produce the next
calibration voltages for each amplifier branch.
FIGS. 16A to 16C illustrate an example in which a combined
calibration and digital predistortion scheme is used in order to
achieve a total radiated adjacent channel power performance in
accordance with an operating communication protocol (e.g. 3GPP/New
Radio). Total radiated adjacent channel power (ACP) is a figure of
merit in which the adjacent channel power is integrated over the
space. Hence, directing the ACP (as illustrated with respect to
FIG. 5C) does not necessarily change the total radiated ACP. This
is because the antenna array fundamentally only directs the power
and so measuring the ACP in one direction does not give an
indication of the total radiated ACP because other directions may
have different amounts of distortion.
FIG. 16A shows two plots superposed on each other. A first plot,
1901, represents a response of a bandpass filter for ACP measures.
A second plot 1902 represents a spectrum in the output of an nth
power amplifier in the transmitter array.
The total radiated adjacent channel power ratio (ACPR) can be
calculated as a difference between the total radiated channel power
(e.g. a sum over channel powers over power amplifier outputs) and
total radiated adjacent channel power (a sum over adjacent channel
powers of power amplifiers). In this arrangement, the calibration
circuitry filters a portion of an adjacent channel power as shown
in FIG. 16A, by RF filters of each power amplifier output and sums
the filtered portions together during the processing. As the
individual power amplifier outputs do not depend on a beamforming
direction, the formed indication represents a total amount of
radiated adjacent channel power over three-dimensional space. This
may be used to control power amplifier responses (such as input
amplitudes, bias voltages, etc.) to cause the responses to be
different enough to each other that over-the-air linearization can
benefit from the differences in their responses. This can help to
improve quality of signal in the transmission direction in addition
to helping to cause compliance with an adjacent channel power total
radiated power requirement of an operating communication protocol.
Therefore, the calibration circuitry may also be used to ensure
compliance with the total radiated adjacent channel power
linearization requirement of a transmission array, such as
discussed above in connection with FIG. 4. This arrangement/method
thus gives a possibility to make a trade-off between total radiated
ACP integrated over the space and quality of signal measured at the
intended transmission direction.
FIG. 16B is a block diagram of the combined calibration and
linearization circuitry. This Figure differs from that of FIG. 6 in
that example components of the measurement unit are
illustrated.
In this example, each output from a transmit path is input to a
respective bandpass filter 1901a', 1901b', . . . 1901n', which in
turn passes the output to a respective received signal strength
indicator 1902a', 1902b', . . . 1902n'. The output from these
received signal strength indicators are output to the calibration
unit.
FIG. 16C presents a block diagram of another arrangement for making
a total radiated ACP measurement.
This Figure differs from that of FIG. 6 in that the measurement
unit comprises a downconverter 1901'' configured to receive any one
input from a plurality of outputs of a transmit path 1902''. This
selection between an output/transmit path may be effected by
switching elements 1903''. The signal output by the downconverter
1901'' is output to a bandpass filter 1904'', which in turn passes
it to a received signal strength indicator 1905'' (after
filtering).
In FIG. 16C, the signal is downconverted by downconverter 1901'' to
a lower frequency before filtering in order to decrease the
requirements of the filter that is filtering the portion of the ACP
for total radiated ACP calculation block. The mixer or mixers in
may be shared with multiple feedback branches by switching elements
1903'' in order to decrease the complexity of the circuit.
It is understood in the above that circuitry for the beam
calibration may receive information indicating the applied
linearization coefficients directly from the digital predistortion
calculation unit. The circuitry for the beam calibration may use
this received information to determine how to calibrate at least
one power amplifier in the array of power amplifiers. It is further
understood that circuitry for the beam calibration may not receive
information indicating the applied linearization coefficients
directly from the digital predistortion calculation unit.
It should be understood that each block of the flowchart of the
Figures and any combination thereof may be implemented by various
means or their combinations, such as hardware, software, firmware,
one or more processors and/or circuitry.
It is noted that whilst embodiments have been described in relation
to one example of a standalone LTE networks, similar principles may
be applied in relation to other examples of standalone 3G, LTE or
5G networks. It should be noted that other embodiments may be based
on other cellular technology other than LTE or on variants of LTE.
Therefore, although certain embodiments were described above by way
of example with reference to certain example architectures for
wireless networks, technologies and standards, embodiments may be
applied to any other suitable forms of communication systems than
those illustrated and described herein.
It is also noted herein that while the above describes example
embodiments, there are several variations and modifications which
may be made to the disclosed solution without departing from the
scope of the present invention.
It should be understood that the apparatuses may comprise or be
coupled to other units or modules etc., such as antennas, radio
parts or radio heads, used in or for transmission and/or reception.
Although the apparatuses have been described as one entity,
different modules and memory may be implemented in one or more
physical or logical entities.
In general, the various embodiments may be implemented in hardware
or special purpose circuits, software, logic or any combination
thereof. Some aspects of the invention may be implemented in
hardware, while other aspects may be implemented in firmware or
software which may be executed by a controller, microprocessor or
other computing device, although the invention is not limited
thereto. While various aspects of the invention may be illustrated
and described as block diagrams, flow charts, or using some other
pictorial representation, it is well understood that these blocks,
apparatus, systems, techniques or methods described herein may be
implemented in, as non-limiting examples, hardware, software,
firmware, special purpose circuits or logic, general purpose
hardware or controller or other computing devices, or some
combination thereof.
The embodiments of this invention may be implemented by computer
software executable by a data processor of the mobile device, such
as in the processor entity, or by hardware, or by a combination of
software and hardware. Computer software or program, also called
program product, including software routines, applets and/or
macros, may be stored in any apparatus-readable data storage medium
and they comprise program instructions to perform particular tasks.
A computer program product may comprise one or more
computer-executable components which, when the program is run, are
configured to carry out embodiments. The one or more
computer-executable components may be at least one software code or
portions of it.
Further in this regard it should be noted that any blocks of the
logic flow as in the Figures may represent program steps, or
interconnected logic circuits, blocks and functions, or a
combination of program steps and logic circuits, blocks and
functions. The software may be stored on such physical media as
memory chips, or memory blocks implemented within the processor,
magnetic media such as hard disk or floppy disks, and optical media
such as for example DVD and the data variants thereof, CD. The
physical media is a non-transitory media.
The memory may be of any type suitable to the local technical
environment and may be implemented using any suitable data storage
technology, such as semiconductor based memory devices, magnetic
memory devices and systems, optical memory devices and systems,
fixed memory and removable memory. The data processors may be of
any type suitable to the local technical environment, and may
comprise one or more of general purpose computers, special purpose
computers, microprocessors, digital signal processors (DSPs),
application specific integrated circuits (ASIC), FPGA, gate level
circuits and processors based on multi core processor architecture,
as non-limiting examples.
Embodiments of the inventions may be practiced in various
components such as integrated circuit modules. The design of
integrated circuits is by and large a highly automated process.
Complex and powerful software tools are available for converting a
logic level design into a semiconductor circuit design ready to be
etched and formed on a semiconductor substrate.
The foregoing description has provided by way of non-limiting
examples a full and informative description of the exemplary
embodiment of this invention. However, various modifications and
adaptations may become apparent to those skilled in the relevant
arts in view of the foregoing description, when read in conjunction
with the accompanying drawings and the appended claims. However,
all such and similar modifications of the teachings of this
invention will still fall within the scope of this invention as
defined in the appended claims. Indeed there is a further
embodiment comprising a combination of one or more embodiments with
any of the other embodiments previously discussed.
* * * * *